A data exchange service or a contents distribution service are in practical use today as one aspect of services using a communication device with a radio communication function. For example, in a network game using portable game terminals, an item obtained in one portable game terminal is passed to another portable game terminal by radio communication. Further, there is a service that distributes advertisement contents from a radio base station placed in a particular store or the like to communication terminals located in close proximity for the purpose of advertisement for a sales promotion.
In such a data exchange service or contents distribution service, a promotion of use of service or an increase in entertainment value are expected by appropriately selecting data to be passed according to the degree of interest of a user, the closeness between users or the like. Then, use of the distance between communication devices that perform radio communication is considered as a parameter to measure the degree of interest of a user or the closeness between users.
The field intensity of a radio signal transmitted from a communication device that performs radio communication is known to be inversely proportional to the square or cube of the distance in close proximity to the communication device being a signal transmission source and inversely proportional to the distance in no proximity in the ideal space. Such characteristics of the field intensity of a radio signal are established also in the wireless LAN (Local Area Network) specifications such as IEEE 802.11b or 11g. A technique of estimating the distance between communication devices that perform radio communication with use of such characteristics of the field intensity of a radio signal is described in Patent Literature 1, for example.    Patent Literature 1: JP 2006-300918 (A)